Maximum Horizontal Stress and Wellbore Stability While Drilling: Modeling and Case Study

Abstract

Abstract Maximum horizontal stress is a critical parameter used in drilling optimization and wellbore stability modeling. Current maximum horizontal stress prediction from wellbore breakout was based on the maximum tangential stress on the wellbore wall to be equal to the rock uniaxial compressive strength. We assume that the vertical, minimum and maximum horizontal stresses define a specific relationship when the stresses in the formation are in equilibrium. Based on a generalized Hooke's law with coupling the equilibrium of stresses and pore pressure, the maximum horizontal stress can be solved using this relationship. This new technique can reduce the uncertainty of in-situ stress prediction by narrowing the area of the conventional polygon of the in-situ stresses. We also propose a new method of the maximum horizontal stress determination from analyses of drilling-induced near-wellbore stresses and breakouts. The near-wellbore stresses are obtained from poroelastic equations. By using Mohr-Coulomb failure criterion, the maximum horizontal stress magnitude can be derived from these equations and from analysis of wellbore breakout obtained from borehole caliper logs. The new technique is compared to the existing methods in an example from measured borehole failures in a case study. These comparisons demonstrate that new technique provides a much better result than the current available methods, because the wellbore failure is related with all of the near-wellbore stresses. A case study has been conducted in a GOM oil field to predict pre-drill wellbore stability, where borehole instability was the main cause of borehole trouble time in offset wells. Using the proposed in-situ stress method, an improved borehole stability model was built to predict the pre-drill mud weight window. Applying this prediction, wellbore failures and drilling risks were greatly reduced.